I have both IL-2 and CFS2, they both have there good sides along with the bad.
CFS2 online kinda sux, lots of kiddies with hacks ruin a good experience, but every once in while youll find a good one, gotta fly those birds at 100% realism. Yes u can download lots of of addons, anything from jets to ufos. if ya get a chance DL and install nibbos addons, adds alot of cool effects, smoke, oil spills, gun smoke, etc.
I think theres even a proggy that will let you fly co-op missions online.
SP is good, AI are pretty good, especially when ya get zeros on your tail, but the inflight commands to other planes in your flight are kinda lame, not enough.
IL-2, well this is by far the best flightsim i have flown, but to get it running smoothly was a Biiiitch!! But well worth the effort.
Stunning scenery along with great AI make me keep coming back for more, I have only started playin this so have yet to try it online, right now it doesnt seem to important, cause im havin to much fun in SP. Have over 100 confirmed air kills and 63 confirmed ground kills, and im only on the 17 mission flying as a Russian Fighter Jock. My Bomber carrer is just getting underway, this is gonna take time, not to much target practice.
Again both are fun but i prefer IL-2 over CFS2